This invention relates to a method and apparatus for controlling electromagnetic transmit power for variable-rate vocoded communications.
Digital communication systems often use vocoders to encode voice information over an air interface. Multi-rate vocoders allow efficient use of the air-interface by reducing the information transmitted during breaks in the conversation or during silence, when no information needs to be conveyed between the mobile station and the base station. Certain multi-rate vocoders can provide rates that are proportional to the amount of speech information to be transmitted. Multi-rate vocoders can also provide rates selected to provide a desired quality of service. Generally, the full-rate or maximum rate of a vocoder corresponds to the highest quality transmission for that vocoder.
The mobile station needs to determine the vocoding rate of a received transmission to decode the transmitted information. In the past, mobile stations have waited until receiving an entire frame (or a sequence of interleaved frames) and then tried decoding the frame in accordance with each possible rate, to determine the vocoding rate. The foregoing iterative decoding approach relies upon trial and error to determine the vocoding rate. At the mobile station, each rate is tried in sequence until the decoding rate matches the transmission rate. For example, if a communication system has four possible vocoding rates, a mobile station may decode the same received frame up to four times, once for each possible rate.
The iterative decoding approach introduces a decoding delay and burdens the processing system of the mobile station. The mobile station may expend significant electrical power, while using up a battery charge, to determine the vocoding rate for each downlink transmission. Although battery life may be extended by using bulkier batteries with higher capacity for mobile stations, such battery changes reduce the portability of mobile stations. Thus, a need exists for determining vocoding rate in a manner that reduces battery consumption and the processing burden of the mobile station.
If the vocoding rate is unknown when a mobile station measures signal energy per bit-to-noise energy density (Eb/No) of a voice channel for power control purposes, Eb/No estimation may be inaccurate or slower than desired to compensate for propagational changes in the downlink transmit signal. For example, the delay associated with the iterative approach is at least one frame duration (e.g., 20 milliseconds) for decoding a received signal at the mobile station. Accordingly, communications system capacity may be reduced by deficient power control where the vocoding rate is not available until after a downlink frame or sequence of frames is received. Thus, a need exists for determining the vocoding rate prior to receiving an entire downlink frame or a sequence of frames to enhance power control performance.
In accordance with the invention, a base station assembles a frame including information bits at a vocoding rate for downlink transmission over a traffic channel as channel bits at a channel rate. The base station places at least one rate-indicating bit at a beginning of the frame for indicating the vocoding rate. The mobile station evaluates the downlink transmission with consideration of the vocoding rate indicated by the at least one rate-indicating bit. The mobile station can determine the vocoding rate by decoding the beginning of the frame, as opposed to using power-hungry, iterative decoding to determine the vocoding rate. Further, the mobile station can control downlink power with power level adjustment data based on a measured performance parameter of a downlink frame or sub-frame, prior to decoding the entire downlink frame or a series of interleaved frames. The base station receives an uplink signal with a power level adjustment data for the downlink signal.